I'm looking to build a dirigible based on the 'Mummy Returns' airship, which is basically a sailing boat of some description slung under an helium filled envelope.

up til now my research has I.D. the boat as some sort of 1700-1800AD sailing boat, possible a Fishing Boat, a Cog or a Caravell (a 1 or 2 sail boat, a galleon is too big). I have found some really nice profile and overhead plans but nothing to help me with the 'ribs' of the hull.

Weight is a major factor so thin balsa is going to be used, I calculate that 1CuM of Helium will give me about 1.113kg total model weight (including deflated envelope), so I would call that my 'upper limit' for this project.